With every email you send out, include a link to a form at the bottom. Through this form, patrons can update their preferences and contact information.
Create a form where current customers can update their information. This can involve new favorite foods, dietary restrictions, updating contact information, and more. Here you could even include boxes for other restaurants. If a customer checks one of these boxes then they are opted-in automatically for communications from that restaurant.
By now, you've created personalized emails for your patrons similar to this flow. Since you are already sending your customers these emails for special meal promotions, a simple link at the bottom of the email would allow them to change their preferences via the form you created.
Once you have added the link to the bottom of the email, continue to send your emails as normal. In the first email you send with the link, you may want to make a point to tell customers that it is now an option.